Air springs, as they are used in the manufacture of motor vehicles for spring mounting wheel suspensions, include essentially a rolling-lobe resilient member. The rolling-lobe resilient member is delimited at one end by a cover plate and, at the other end, by a roll-off piston and so encloses an air spring volume within its interior space. The rolling-lobe flexible member itself comprises rubber or a rubber-like plastic having attachment beads at its respective ends. The end of the rolling-lobe flexible member disposed at the cover plate is usually attached by flanging and, for this reason, the upper cover plate is also known as a flanged plate.
A flanged plate of this kind mostly includes an essentially planar base and an annularly-shaped rim arranged perpendicularly to the base. The transition from the base to the rim of the flanged plate is provided by a convex arc portion having a radius clearly less than the perpendicular extent of the rim.
The flanging procedure itself corresponds to the swage flanging known from form shaping technology. The flanged plate is disposed with its opening facing downwardly in an annularly-shaped closed lower flanging tool open upwardly. The lower part of the lower flanging tool has a nose-shaped projecting annular shoulder whose diameter is less than the diameter of the rim of the flanged plate. Referring to FIG. 3, a flanging tool is shown schematically. The flanged plate is identified by reference numeral 1 and the lower flanging tool by 2 and the upper flanging tool by 3. The nose-shaped annular shoulder is identified by reference numeral 4.
When flanging, the flanged plate is pressed from above by the upper flanging tool downwardly into the lower flanging tool. The lower edge of the rim of the flanged plate is bent inwardly by the nose-shaped annular shoulder of the lower flanging tool. The rim assumes the form of a torus which includes the upper-end attachment bead of a rubber rolling-lobe flexible member so that a tight and secure connection is established between the rolling-lobe flexible member and the flanged plate.
FIG. 4 shows a detail of a completed flanged rolling-lobe air spring 5 and shows the result of such a flanging procedure. The rim 6 of the flanged plate 1 is formed to have a torus shape about the bead 7 of a rolling-lobe flexible member 8. The base 9 of the flanged plate 1 is planar.
The rim is pressed back toward the base by the pressure exerted when pressing on the rim of the flanged plate. This operation is shown in FIG. 5. The flanged plate 1 is pressed against the upper flanging tool 3 by the force F. In this way, a material flow into the base 9 of the flanged plate 1 results which leads to the situation that the base 9 of the flanged plate 1 bulges to be concave or convex and thereby makes the flanged plate 1 unusable. In order to avoid this, and according to FIG. 6, the upper flanging tool 3 is usually equipped with holding grooves 10 defining sharp edges which, at the beginning of the pressing, dig into the surface of the flanged plate and so prevent the disadvantageous material flow. However, the surface of the flanged plate 1 is damaged which operates disadvantageously on a corrosion protection layer of the flanged plate 1 because this layer is penetrated by the sharp edges defined by the holding grooves 10. An organic corrosion protection layer is therefore completely ineffective at this location and therefore cannot be used.